Document feature flag process

This records the feature flag process we intend to use with Riot and also how
that interacts with other teams and configuration.

Fixes https://github.com/vector-im/riot-web/issues/11116

docs/config.md

@@ -22,9 +22,10 @@ For a good example, see https://riot.im/develop/config.json.
      `default_hs_url` is specified. When multiple sources are specified, it is unclear
      which should take priority and therefore the application cannot continue.
    * As of Riot 1.4.0, identity servers are optional. See [Identity servers](#identity-servers) below.
-1. `features`: Lookup of optional features that may be `enable`d, `disable`d, or exposed to the user
-   in the `labs` section of settings.  The available optional experimental features vary from
-   release to release. The available features are described in [labs.md](labs.md).
+1. `features`: Lookup of optional features that may be `enable`d, `disable`d, or
+   exposed to the user in the `labs` section of settings. The available
+   optional experimental features vary from release to release and are (usually) [documented](labs.md). The feature flag process is
+   [documented](feature-flags.md) as well.
 1. `showLabsSettings`: Shows the "labs" tab of user settings even when no `features` are enabled
    or present. Useful for getting at settings which may be otherwise hidden.
 1. `brand`: String to pass to your homeserver when configuring email notifications, to let the

docs/feature-flags.md

@@ -0,0 +1,89 @@
+# Feature flags
+
+When developing new features for Riot, we use feature flags to give us more
+flexibility and control over when and where those features are enabled.
+
+For example, flags make the following things possible:
+
+* Extended testing of a feature via labs on develop
+* Enabling features when ready instead of the first moment the code is released
+* Testing a feature with a specific set of users (by enabling only on a specific
+  Riot instance)
+
+The size of the feature controlled by a feature flag may vary widely: it could
+be a large project like reactions or a smaller change to an existing algorithm.
+A large project might use several feature flags if it's useful to control the
+deployment of different portions independently.
+
+Everyone involved in a feature (engineering, design, product, reviewers) should
+think about its deployment plan up front as best as possible so we can have the
+right feature flags in place from the start.
+
+## Interaction with spec process
+
+Historically, we have often used feature flags to guard client features that
+depend on unstable spec features. Unfortunately, there was never clear agreement
+about how long such a flag should live for, when it should be removed, etc.
+
+Under the [new spec
+process](https://github.com/matrix-org/matrix-doc/pull/2324), server-side
+unstable features can be used by clients and enabled by default as long as
+clients commit to doing the associated clean up work once a feature stabilises.
+
+## Starting work on a feature
+
+When starting work on a feature, we should create a matching feature flag:
+
+* Add a new
+  [setting](https://github.com/matrix-org/matrix-react-sdk/blob/develop/src/settings/Settings.js)
+  of the form:
+```js
+    "feature_cats": {
+        isFeature: true,
+        displayName: _td("Adds cats everywhere"),
+        supportedLevels: LEVELS_FEATURE,
+        default: false,
+    },
+```
+* Check whether the feature is enabled as appropriate:
+```js
+    SettingsStore.isFeatureEnabled("feature_cats")
+```
+* Add the feature to the [set of labs on develop](../riot.im/develop/config.json):
+```json
+    "features": {
+        "feature_cats": "labs"
+    },
+```
+* Document the feature in the [labs documentation](labs.md)
+
+With these steps completed, the feature is disabled by default, but can be
+enabled on develop by interested users for testing.
+
+Different features may have different deployment plans for when to enable where. The
+following lists a few common options.
+
+## Enabling by default on develop
+
+Set the feature to `enable` in the [develop config](../riot.im/develop/config.json):
+
+```json
+    "features": {
+        "feature_cats": "enable"
+    },
+```
+
+## Enabling by default on staging and app
+
+Set the feature to `enable` in the [app config](../riot.im/app/config.json).
+
+## Feature deployed successfully
+
+Once we're confident that a feature is working well, we should remove the flag:
+
+* Remove the [setting](https://github.com/matrix-org/matrix-react-sdk/blob/develop/src/settings/Settings.js)
+* Remove all `isFeatureEnabled` lines that test for the feature's setting
+* Remove the feature from the [labs documentation](labs.md)
+* Remove feature state from [develop](../riot.im/develop/config.json) and
+  [app](../riot.im/app/config.json) configs
+* Celebrate! 🥳